Why does changing an Array in JavaScript affect co

Question

I've written the following JavaScript:

var myArray = ['a', 'b', 'c'];
var copyOfMyArray = myArray;
copyOfMyArray.splice(0, 1);
alert(myArray); // alerts ['b','c']
alert(copyOfMyArray); // alerts ['b','c']

var myNumber = 5;
var copyOfMyNumber = myNumber;
copyOfMyNumber = copyOfMyNumber - 1;
alert(myNumber); // alerts 5
alert(copyOfMyNumber); // alerts 4        

This code declares a variable myArray and sets it to an array value. It then declares a second variable copyOfMyArray and sets it to myArray. It performs an operation on copyOfMyArray and then alerts both myArray and copyOfMyArray. Somehow, when I perform an operation on copyOfMyArray, it appears that the same operation is performed on myArray.

The code then does the same thing with a number value: It declares a variable myNumber and sets it to a number value. It then declares a second variable copyOfMyNumber and sets it to myNumber. It performs an operation on copyOfMyNumber and then alerts both myNumber and copyOfMyNumber. Here, I get the expected behavior: different values for myNumber and copyOfMyNumber.

What is the difference between an array and a number in JavaScript that it seems changing an array changes the value of a copy of the array, where as changing a number does not change the value of a copy of the number?

I'm guessing that for some reason, the array is referred to by reference and the number by value, but why? How can I know what behavior to expect with other objects?

Answer 1

Well, the only possible answer — and the correct one — is that you're not actually copying the array. When you write

var copyOfArray = array;

you're assigning a reference to the same array into another variable. They're both pointing at the same object, in other words.

Answer 2

You can add some error handling depending on your cases and use something similar to the following function to solve the issue. Please comment for any bugs / issues / efficiency ideas.

function CopyAnArray (ari1) {
   var mxx4 = [];
   for (var i=0;i<ari1.length;i++) {
      var nads2 = [];
      for (var j=0;j<ari1[0].length;j++) {
         nads2.push(ari1[i][j]);
      }
      mxx4.push(nads2);
   }
   return mxx4;
}